Cadmium mercury telluride, Hg1-xCdxTe, is well known as a material for use in infrared devices, such as detectors, sources, LEDs, negative luminescence devices etc. Cadmium mercury telluride, referred to as CMT (or sometimes mercury cadmium telluride—MCT) is a semiconductor alloy, the bandgap of which can be varied by altering the composition of the alloy, i.e. the cadmium content x. The band gap may be tuned so that CMT can be used for a range of infrared devices covering short wave (SW), medium wave (MW), long wave (LW) and very long wave (VLW) infrared wavelengths. CMT is the material of choice for many infrared focal plane array applications. Low leakage current and high carrier mobility result in detectors with excellent sensitivity. CMT is the best solution for single and multi-band systems covering a wide range of wavelengths because it is possible to tune the wavelength by selecting the appropriate composition and it is possible to design and grow structures where the composition is tuned so that two or more wavelengths are operational within a single device.
The general principle for fabricating infrared devices is well established. CMT is grown epitaxially onto a crystalline substrate. Devices are then formed by mesa etching, ion implantation or ion beam milling. Metal contacts are then formed and the devices bonded to a silicon read out circuit. Note that CMT is also grown as bulk crystals from which devices are made by ion implantation or ion beam milling but epitaxial growth can be advantageous over bulk crystal growth.
Various epitaxial growth methods have been suggested for fabricating CMT. Metal-organic vapour phase epitaxy (MOVPE) has been successfully used as a technique for reproducible and uniform growth over large areas. U.S. Pat. No. 4,650,539 describes manufacture of CMT using MOVPE. U.S. Pat. No. 4,566,918 is a modification of this technique which grows thin layers of CdTe and HgTe which interdiffuse to form a uniform CMT structure. U.S. Pat. No. 4,950,621 describes a MOVPE technique for CMT growth which uses a photocatalytic decomposition of the metal-organic compounds.
Other methods for growing CMT include molecular beam epitaxy (MBE). Infrared devices have been formed from CMT grown on a cadmium zinc telluride (Cd1-yZnyTe also known as CZT) substrate by an MBE process. See for example; M Zandian, J D Garnett, R E Dewames, M Carmody, J G Pasko, M Farris, C A Cabelli, D E Cooper, G Hildebrandt, J Chow, J M Arias, K Vural and D N B Hall, J. Electronic Materials 32(7) 803 (2003) “Mid-wavelength infrared p-on-n Hg1-xCdxTe heterostructure detectors: 30-120 Kelvin state of the art performance”, or J D Phillips, D D Edwall and D L Lee J. Electronic Materials 31(7) 664 (2002) “Control of very long wavelength infrared HgCdTe detector cut-off wavelength”.
Infrared imaging applications increasingly demand large area, two dimensional detector arrays for long range detection and identification. As the physical size of these arrays has increased so the limitations of traditional substrate materials and growth techniques for CMT have become apparent. Cadmium zinc telluride has been widely used as a substrate for CMT growth but is only available in small sizes which limits its usefulness for the production of large arrays. Cadmium telluride, which has likewise been used as a substrate, is also only available in small sizes. Further, both CdTe and CZT are extremely fragile and the crystal quality is not particularly good.
Gallium arsenide (GaAs) substrates are available in relatively large sizes. However as mentioned the devices are usually bonded to a silicon read-out circuit. In operation detectors are often cooled to low temperatures, for example around 80K (although different devices work best at different temperatures) to reduce thermal noise. At the operating temperatures of the detectors the thermal mismatch between the silicon read out circuit and GaAs substrate can cause delamination of the infrared devices from the circuitry. This effect can be reduced by thinning the substrate but thinning processes can be complex, reducing yields and increasing production costs. This problem of thermal mismatch also applies to cadmium telluride and CZT substrates.
Silicon has been proposed as a substrate because a silicon substrate would inherently be thermally matched to the read out circuitry.
MBE techniques have been applied to growing CMT on silicon, growing buffer layers on the silicon prior to CMT growth, for example;—T J de Lyon, J E Jensen, M D Gorwitz, C A Cockrum, S M Johnson and G M Venzor, J. Electronic Materials 28, 705 (1999). MBE growth of CMT on silicon has proved a challenging task. Firstly, for MBE growth of CMT on any substrate the growth temperature must be accurately controlled, requiring reproducible wafer mounting techniques and fine substrate temperature control. Secondly, material defects have proved difficult to eliminate. These defects do not always have a severe effect on medium wavelength infrared device characteristics (depending on the device) but they do detrimentally affect long-wavelength devices. Consequently growth of CMT on silicon by MBE is a difficult process and has only produced acceptable mid-wavelength infrared devices and arrays.
MOVPE growth of CMT on silicon to produce working devices has also been problematic. See J. Electronic Materials 25(8) (1996) page 1347 K Shigenaka, K Matsushita, L Sugiura, F Nakata and K Hirahara, M Uchikoshi, M Nagashima and H Wada “Orientation dependence of HgCdTe epitaxial layers grown by MOCVD on silicon substrates”,
page 1353 K Maruyama, H Nishino, T Okamoto, S Murakami, T Saito, Y Nishijima, M Uchikoshi, M Nagahima and H Wada “Growth of (111) HgCdTe on (100) Si by MOVPE using metal organic tellurium absorption and annealing”, or Page 1358H Ebe, T Okamoto, H Nishino, T Saito and Y Nishijima, M Uchikoshi, M Nagashima and H Wada “Direct growth of CdTe on (100), (211), and (111) Si by metal organic chemical vapour deposition”.
More recently it has been reported that CMT can be produced on a silicon substrate by growing buffer layers on the substrate by MBE and then growing CMT by MOVPE; “Long wavelength infrared focal plane arrays fabricated from HgCdTe grown on silicon substrates”, D J Hall, L Buckle, N T Gordon, J Giess, J E Hails, J W Cairns, R M Lawrence, A Graham, R S Hall, C Maltby and T Ashley Presented at Defense and Security Symposium 2004 (Formerly AeroSense) 12-16 Apr. 2004 Gaylord Palms Resort and Convention Center Orlando (Kissimmee), Fla. USA, Conference proceedings in press and “High performance long-wavelength HgCdTe infrared detectors grown on silicon substrates”, D J Hall, L Buckle, N T Gordon, J Giess, J E Hails, J W Cairns, R M Lawrence, A Graham, R S Hall, C Maltby and T Ashley, Applied Physics Letters Volume 85, Issue 11, pp. 2113-2115.
This technique allows the growth of CMT onto silicon which can then be processed for device formation and bonded to read out circuitry.
However this still requires that, prior to bump bond hybridisation, the wafers of the detector arrays and read out integrated circuits (ROICs) are sawn into individual components ready for the hybridisation process. This is an expensive technique and sawing wafers and bonding can be relatively low yield.
International Patent Application WO02/084741A2 describes a monolithic infrared sensing device where CMT layers are grown directly on a silicon read out circuit. The method described in this patent application involves forming a growth window on the silicon substrate in which is grown a buffer layer of cadmium telluride and then layers of CMT by MBE. This again however relies on MBE growth of CMT on silicon with the attendant difficulties.